A non-specific aminopeptidase from Aspergillus

摘要

A fermentation broth supernatant of the Aspergillus oryzae strain ATCC20386 contains aminopeptidase activity that releases a wide variety of amino acids from natural peptides. The supernatant was fractionated by anion exchange chromatography. Based on the primary amino acid sequence data obtained from proteins in certain fractions, polymerase chain reaction (PCR) primers were made and a PCR product was generated. This PCR product was used to screen an A. oryzae cDNA library from which the full length gene was then obtained. Fusarium venenatum and A. oryzae were used as hosts for gene expression. Transformed strains of both F. venenatum and A. oryzae over-expressed an active aminopeptidase (E. C. 2.4.11), named aminopeptidase II. The recombinant enzyme from both fungal hosts appeared as smears on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After deglycosylation of the N-linked sugars, both samples were a sharp band at ～ 56 kDa and had identical N-terminal amino acid sequences. Aminopeptidase II is a metalloenzyme with, presumably, Zn in the active site. Using various natural peptides and para-nitroanilides (pNAs) of amino acids as substrates, the aminopeptidase was found to be non-specific. Only X-Pro bonds demonstrated resistance to hydrolysis catalyzed by this aminopeptidase. The optimal enzyme activity was observed at pH 9.5 and 55 ℃. Among amino acid pNAs, Leu-pNA appears to have the highest value of bimolecular constant of 40 min~(-1) mM~(-1) (k_(cat) = 230 min~(-1); K_m = 5.8 mM) at pH 7.5 and 21 ℃. Among Xaa-Ala-Pro-Tyr-Lys-amide pentapeptides, the velocity of catalytic hydrolysis at pH 7.5 and 21 ℃ was in a decreasing order: Pro, Ala, Leu, Gly and Glu.